The invention relates to a method for grinding a rotationally symmetrical machine part provided with a longitudinal bore, a one end-face surface of which is embodied as an active surface in the form in particular of a flat truncated cone with a cross-section with a straight or curved contour.
The machine parts to be ground with this method are present for instance in transmissions with continuously variable gears, as are needed in motor vehicles. Two machine parts oppose one another with active surfaces facing one another. The active surfaces thus form an annular space with a nearly wedge-shaped cross-section in which a tension member, such as for instance a chain or a belt, moves in and out between different radii depending on a distance from the active surfaces. Since such a transmission must work very precisely and transmit large torques, high demands are placed on the dimensional stability and surface quality of the machine parts. This also applies to the associated grinding procedures, in particular when grinding the active surface.
In accordance with the prior art known from commercial practice, the method cited in the foregoing has been performed in single operations, that is, in a plurality of clampings. The active surface is ground by means of corundum grinding wheels using the angular infeed grinding method. For interior cylindrical grinding of a longitudinal bore located on the machine part, the machine part must then be clamped in another machine, where the internal cylindrical grinding of the bore wall can occur using an appropriate small grinding wheel.
The known method has a number of disadvantages. First, it requires grinding wheels with a conical shape or with a highly graduated diameter, which are difficult to manufacture and dress. In such grinding wheels with circumferential regions of very different diameters, the circumferential speeds of the regions to be ground are also different. This means that the critical cutting speed at the grinding location must be different and therefore cannot be optimal over all. The result of this are regions of varying roughness, which has a negative effect on the active surface. Finally, there are also problems involving cooling by means of the conventional emulsions and grinding oils. That is, during angular infeed grinding a narrowing wedge occurs at the grinding location, and coolant/lubricant cannot be fed to it optimally. The result is thus uneven cooling of the grinding location. All of these difficulties can be traced back to the fact that the aforesaid known method has in the past been performed with corundum grinding wheels, which have a significantly shorter service life and must be dressed more frequently than CBN grinding wheels, which have since come into wide use.
DD 143 700 concerns an apparatus for grinding tungsten plates that are used for instance as rotating electrodes in x-ray tubes. According to the drawing, such a tungsten plate has the contour of a truncated cone in which the incline of the surface line is approximately 30° relative to the base. In this known apparatus, the tungsten plate is clamped in a workpiece holder that is pivotable about an axis perpendicular to the apparatus frame. Situated opposing the workpiece holder is a longitudinal support that is displaceable in the horizontal plane. Arranged on the longitudinal support is a compound slide rest that carries a grinding spindle for driving a small cylindrical grinding wheel that acts for internal grinding of a bore in the tungsten plate. Separated from this compound slide rest, the longitudinal support furthermore carries a rigid electrogrinding spindle for driving a conical grinding wheel. One end face and the cone envelope-shape region of the tungsten plate is to be ground with the conical grinding wheel. For this, the conical grinding wheel and the tungsten plate must be brought into the correct position relative to one another by pivoting the workpiece holder, displacing the longitudinal support, and using manually actuated advancing controls.
Nothing other than angled grinding in the region of the cone envelope can be done by DD 143 700. The known apparatus, which must in part be operated manually, is difficult to operate and requires some skill.
Known from EP 1 022 091 A2 is a tool machine for grinding workpieces in which two cylindrical grinding wheels of different sizes are situated on one turret that is itself arranged on a displaceable slide. By pivoting the turret 180°, the two grinding wheels can be selectively brought up against different regions of a rotationally symmetrical workpiece. The workpiece is arranged in a workpiece receiver that is itself displaceable in the longitudinal direction of the workpiece. For grinding, the workpiece is rotated. In addition, in this known workpiece machine the workpiece receiver can be adjusted about an angle of +/−30° inclined to the displacement direction of the workpiece receiver. EP 1 022 091 A2 does not explain how grinding should proceed when the workpiece receiver is in an angled position. However, since pivoting of the turret carrying the grinding wheel is expressly indicated in increments of 90°, it is obvious that with this known tool machine, as well, longitudinal grinding with one grinding wheel is intended when conical exterior contours with significant angles of inclination in the cone are to be ground.